I adopted Stomps when he was approximately 3-5 years old, and after about 8 months of being in my house he developed aggression problems with my other dog, Lightning. (I doubt if the rescue knew anything about him biting--usually dogs don't display the type of behavior you describe until they've lived somewhere long enough to establish it--and the people they live with--as their territory.) While the circumstances surrounding your case and mine differ, the root problem does not: Your dog thinks he's at the top of the heirarchy, and he can push everyone around. Your trainer should be able to help you, but in the meantime, here are some things you can implement to get your dog's behavior in check. First of all, don't let him sleep in anyone's bed. That causes more dominance problems than just about anything I've seen. With Stomps, I can't even let him sit in a chair with me, because he becomes very jealous if Lightning approaches. Have your mother, your kids, everyone who comes in contact with the dog on a regular basis put him through his obedience paces. In essence, you're putting him in his place, which is below all the people he comes in contact with. That will help with his dominance issues. If he is biting because he's being startled, then also do as Terry advised. But I think it's a combination of dominance and fear.